[PATCH] linux: spawni.c: simplify error reporting to parent

[Rasmus Villemoes <rv@rasmusvillemoes.dk>]

Using VFORK already ensures that the parent does not run until the
child has either exec'ed succesfully or called _exit. Hence we don't
need to read from a CLOEXEC pipe to ensure proper synchronization - we
just make explicit use of the fact the the child and parent run in the
same VM, so the child can write an error code to a field of the
posix_spawn_args struct instead of sending it through a pipe.

To ensure that this mechanism really works, the parent initializes the
field to -1 and the child writes 0 before execing.

This eliminates some annoying bookkeeping that is necessary to avoid
the file actions from clobbering the write end of the pipe, and
getting rid of the pipe creation in the first place means fewer system
calls (four in the parent, usually one in the child) and fewer
chanches for the spawn to fail (e.g. if we're close to EMFILE).

Signed-off-by: Rasmus Villemoes <rv@rasmusvillemoes.dk>
---
 sysdeps/unix/sysv/linux/spawni.c | 71 ++++++++++++++--------------------------
 1 file changed, 25 insertions(+), 46 deletions(-)

diff --git a/sysdeps/unix/sysv/linux/spawni.c b/sysdeps/unix/sysv/linux/spawni.c
index bb3eecf..c0e15ac 100644
--- a/sysdeps/unix/sysv/linux/spawni.c
+++ b/sysdeps/unix/sysv/linux/spawni.c
@@ -17,6 +17,7 @@
    <http://www.gnu.org/licenses/>.  */
 
 #include <spawn.h>
+#include <assert.h>
 #include <fcntl.h>
 #include <paths.h>
 #include <string.h>
@@ -44,11 +45,12 @@
    3. Child must synchronize with parent to enforce 2. and to possible
       return execv issues.
 
-   The first issue is solved by blocking all signals in child, even the
-   NPTL-internal ones (SIGCANCEL and SIGSETXID).  The second and third issue
-   is done by a stack allocation in parent and a synchronization with using
-   a pipe or waitpid (in case or error).  The pipe has the advantage of
-   allowing the child the communicate an exec error.  */
+   The first issue is solved by blocking all signals in child, even
+   the NPTL-internal ones (SIGCANCEL and SIGSETXID).  The second and
+   third issue is done by a stack allocation in parent, and by using a
+   field in struct spawn_args where the child can write an error
+   code. CLONE_VFORK ensures that the parent does not run until the
+   child has either exec'ed successfully or exited.  */
 
 
 /* The Unix standard contains a long explanation of the way to signal
@@ -79,7 +81,6 @@
 
 struct posix_spawn_args
 {
-  int pipe[2];
   sigset_t oldmask;
   const char *file;
   int (*exec) (const char *, char *const *, char *const *);
@@ -89,6 +90,7 @@ struct posix_spawn_args
   ptrdiff_t argc;
   char *const *envp;
   int xflags;
+  int err;
 };
 
 /* Older version requires that shell script without shebang definition
@@ -125,11 +127,8 @@ __spawni_child (void *arguments)
   struct posix_spawn_args *args = arguments;
   const posix_spawnattr_t *restrict attr = args->attr;
   const posix_spawn_file_actions_t *file_actions = args->fa;
-  int p = args->pipe[1];
   int ret;
 
-  close_not_cancel (args->pipe[0]);
-
   /* The child must ensure that no signal handler are enabled because it shared
      memory with parent, so the signal disposition must be either SIG_DFL or
      SIG_IGN.  It does by iterating over all signals and although it could
@@ -203,17 +202,6 @@ __spawni_child (void *arguments)
 	{
 	  struct __spawn_action *action = &file_actions->__actions[cnt];
 
-	  /* Dup the pipe fd onto an unoccupied one to avoid any file
-	     operation to clobber it.  */
-	  if ((action->action.close_action.fd == p)
-	      || (action->action.open_action.fd == p)
-	      || (action->action.dup2_action.fd == p))
-	    {
-	      if ((ret = __dup (p)) < 0)
-		goto fail;
-	      p = ret;
-	    }
-
 	  switch (action->tag)
 	    {
 	    case spawn_do_close:
@@ -273,6 +261,7 @@ __spawni_child (void *arguments)
   __sigprocmask (SIG_SETMASK, (attr->__flags & POSIX_SPAWN_SETSIGMASK)
 		 ? &attr->__ss : &args->oldmask, 0);
 
+  args->err = 0;
   args->exec (args->file, args->argv, args->envp);
 
   /* This is compatibility function required to enable posix_spawn run
@@ -280,14 +269,13 @@ __spawni_child (void *arguments)
      (2.15).  */
   maybe_script_execute (args);
 
-  ret = -errno;
-
 fail:
-  /* Since sizeof errno < PIPE_BUF, the write is atomic. */
-  ret = -ret;
-  if (ret)
-    while (write_not_cancel (p, &ret, sizeof ret) < 0)
-      continue;
+  /* errno should have an appropriate non-zero value; otherwise,
+     there's a bug in glibc or the kernel.  For lack of an error code
+     (EINTERNALBUG) describing that, use ECHILD.  Another option would
+     be to set args->err to some negative sentinel and have the parent
+     abort(), but that seems needlessly harsh.  */
+  args->err = errno ? : ECHILD;
   _exit (SPAWN_ERROR);
 }
 
@@ -304,9 +292,6 @@ __spawnix (pid_t * pid, const char *file,
   struct posix_spawn_args args;
   int ec;
 
-  if (__pipe2 (args.pipe, O_CLOEXEC))
-    return errno;
-
   /* To avoid imposing hard limits on posix_spawn{p} the total number of
      arguments is first calculated to allocate a mmap to hold all possible
      values.  */
@@ -333,15 +318,14 @@ __spawnix (pid_t * pid, const char *file,
   void *stack = __mmap (NULL, stack_size, prot,
 			MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
   if (__glibc_unlikely (stack == MAP_FAILED))
-    {
-      close_not_cancel (args.pipe[0]);
-      close_not_cancel (args.pipe[1]);
-      return errno;
-    }
+    return errno;
 
   /* Disable asynchronous cancellation.  */
   int cs = LIBC_CANCEL_ASYNC ();
 
+  /* Child must set args.err to something non-negative - we rely on
+     the parent and child sharing VM.  */
+  args.err = -1;
   args.file = file;
   args.exec = exec;
   args.fa = file_actions;
@@ -355,9 +339,8 @@ __spawnix (pid_t * pid, const char *file,
 
   /* The clone flags used will create a new child that will run in the same
      memory space (CLONE_VM) and the execution of calling thread will be
-     suspend until the child calls execve or _exit.  These condition as
-     signal below either by pipe write (_exit with SPAWN_ERROR) or
-     a successful execve.
+     suspend until the child calls execve or _exit.
+
      Also since the calling thread execution will be suspend, there is not
      need for CLONE_SETTLS.  Although parent and child share the same TLS
      namespace, there will be no concurrent access for TLS variables (errno
@@ -365,22 +348,18 @@ __spawnix (pid_t * pid, const char *file,
   new_pid = CLONE (__spawni_child, STACK (stack, stack_size), stack_size,
 		   CLONE_VM | CLONE_VFORK | SIGCHLD, &args);
 
-  close_not_cancel (args.pipe[1]);
-
   if (new_pid > 0)
     {
-      if (__read (args.pipe[0], &ec, sizeof ec) != sizeof ec)
-	ec = 0;
-      else
-	__waitpid (new_pid, NULL, 0);
+      ec = args.err;
+      assert (ec >= 0);
+      if (ec != 0)
+	  __waitpid (new_pid, NULL, 0);
     }
   else
     ec = -new_pid;
 
   __munmap (stack, stack_size);
 
-  close_not_cancel (args.pipe[0]);
-
   if ((ec == 0) && (pid != NULL))
     *pid = new_pid;
 
-- 
2.1.4